A Treaty is an agreement under international law entered into by actors in international law, namely states and international organizations. A Treaty may also be known as: (international) agreement, protocol, covenant, convention, exchange of letters, etc. Regardless of the terminology, all of these international agreements under international law are equally treaties and the rules are the same. (Note that in United States constitutional law, the term "treaty" has a special meaning which is more restricted than its meaning in international law; see below.)
Treaties can be loosely compared to contracts: both are means of willing parties assuming obligations among themselves, and a party to either that fails to live up to their obligations can be held liable under international law for that breach. The central principle of treaty law is expressed in the maxim pacta sunt servanda—"pacts must be respected".